How do i get my girl to move in with me?

we been going out for almost 3 years. her living situation is terrible and her grandma is trying to "conserve" on bills so she limits my girlfriend to the things her and her mom can do. i.e. showers, heat, electricity. she knows that it is better to move with me but she doesnt wanna leave her mom alone with her gma. any advice on what i can do or say to her.